Reference electrodes are employed in many electrochemical processes to sense the potential of a solution in an electrolytic cell. The sensed potential is often employed to control the operation of the cell potentiostatically.
One type of electrolytic apparatus to which the invention has specifically been applied is an electrochromic display of the kind employing an electrochromic substance in solution which is transparent while dissolved, but colored when electrodeposited upon an electrode. The colored and transparent states form a redox pair so that the deposited colored material can be electrolytically removed by reversing the current direction. One well known substance of this type is the 1,1'-di-heptyl-4,4'-bipyridinium di-cation which is one of the class of electrochromic substances known as the viologens. Transparent in solution, it can be reduced electrochemically to the radical cation which is violet colored. In the presence of a suitable anion such as bromide, phosphate or phosphate/hypophosphite mixtures, the colored viologen radical salt precipitates out on the cathode.
In order to control the write and erase operation of this type of display, it is known to provide in addition to display and counter electrodes, a reference electrode which senses the potential of the solution. Such a reference electrode can be used to control both write and erase operations depending on the particular control scheme selected.
In one known method of controlling a display, selected display electrodes are written to a predetermined contrast by employing a constant current source for a fixed period of time. Under these conditions, a fixed charge is passed and a fixed amount of material is deposited. If the deposit remained absolutely stable upon the display electrodes and conditions remained unchanged, the written display electrodes could be erased by passing an opposite sense constand current for the same period of time. However, many electrochromic deposits slowly redissolve with the consequence that the electrode would be overerased. The forcing of a constant current after all the electrochromic material has been removed would drive the display electrodes more anodic. Depending on the particular materials employed, this could lead to irreversible electrolytic damage to the display electrodes or to damage by liberation of gases within the cell.
Accordingly, the technique of potentiostatic control of erasure has been employed whereby the potential of the counter electrode is controlled with respect to the solution potential as sensed by a reference electrode in the vicinity of the display electrodes. In the method most commonly employed, the reference electrode potential is compared with a predetermined potential corresponding to substantially complete erasure of the display electrodes and the result of the comparison used to control the potential of the counter electrode. Erase current is thus passed through the cell until the reference electrode potential has dropped to the predetermined level. By allowing a small safety margin, over erasure is prevented. The use of reference electrodes in this way is described in a review article entitled "Electrochromic Displays" (New Electronics, Sept. 16, 1975, page 66).
Another use of reference electrodes is to control the write process by maintaining a threshold potential sufficient for the reduction (or oxidation) of the electrochromic substance. Such a use is described in UK Pat. No. 1376799 (Philips) and U.S. Pat. No. 3,950,077 (Jasinski, Texas Instruments).
U.S. Pat. No. 3,950,077 is primarily concerned with overcoming the alleged disadvantage of a reference electrode that an external potential regulating circuit is required. It proposes a non-polarizable counter electrode which is a lead/lead-phosphate half cell. The potential of such a counter electrode does not vary with respect to the solution as would a simple metallic counter electrode. Because of this the counter electrode potential accurately determines the potential at the display electrode and the need for a reference electrode is avoided.
Our U.S. Pat. Nos. 4,167,309 and 4,167,308 and our published U.K. patent application No. 2038065A corresponding to U.S. Pat. No. 4,256,380 also discuss the limitations of reference electrodes in large displays. These are essentially that, since the reference electrode cannot represent the solution potential over the whole area of the display electrodes, the large variation in potential drop through the solution between different display electrodes and the counter electrode will cause uneven writing and erasure of the display electrodes. One of the patents, U.S. Pat. No. 4,167,309 proposes a reticulate counter electrode covering the whole area of the display which is pre-charged to stabilize its potential. One of several ways of precharging the counter electrode is to charge it with the electrochromic substance (viologen) itself. The redox reaction of the viologen at the counter electrode then acts to stabilize its potential with respect to the solution.
These patent applications acknowledge that the drawbacks of reference electrodes are only severe with large area displays. With small displays, of the order of a few centimeters in width, there is no real alternative to the use of a reference electrode as the counter electrodes are too small to maintain their charge for display operations and attempts to increase their area would compromise the visibility of the display electrodes.
References to the actual nature of the reference electrode in the prior art are scanty. The implication is that any conductor will do. The Philips UK Pat. No. 1376799 suggests that the reference electrodes may be of the "same material as the image electrodes" or alternatively can be made from "glass, calomel or the like."
The suggestions as to appropriate reference electrodes in the prior art have been found to be inadequate for a practical small display. A standard calomel reference electrode is a large cumbersome half cell which would have to be remote from the display cell and draw fluid from it by a capillary tube. The simpler prospect of using an electrode similar to the display electrode or a simple wire proves unsatisfactory in that the potential of the electrode is unstable with respect to the solution.
Experimental studies have demonstrated that the potential of a silver wire electrode in a viologen display varies and will drift over a period of time. Such variations can be caused by capacitive and leakage currents in the electrode or by impurities in the solution which react with the silver electrode.
Although the potential of a small reference electrode can be stabilized by deposition of a sufficient charge of electrochromic material thereon, this of itself does not provide a practical reference electrode since such deposits dissolve away.
These considerations, although particularly applicable to electrochromic displays, are also relevant to other electrolytic apparatus employing a reference electrode.